The compartment on the right is for daily use storage, like food, water treatment, replacement filter socks, etc. which is why I painted it to match. The rest on the inside is mechanical access and won't be opened often so I felt it didn't need paint. Working on the doors, and I may have to re-do some things with the canopy before I paint it.
I would still consider painting the inside of the stand to help repel incidental moisture or occassional spills. A rubber shower pan liner, cut to fit, would also reduce any damage to your home in the event of a sump malfunction incident.